This invention relates in general to xerography and more specifically to an imaging member comprising a novel charge transport layer. The invention further relates to a novel electrophotographic system containing a cleaning blade, particularly a doctor blade.
In recent years, interest has been shown in electrophotographic drums for use in high speed office copying machines. Some of these drums are multilayered devices comprising a conductive substrate layer, a blocking interface layer, an optional adhesive layer, a charge generation layer and a charge transport layer. The charge transport layer comprises an organic charge transport molecule dissolved in a polymeric matrix material. This layer is substantially nonabsorbing in the spectral region of intended use, e.g., visible light, but is "active" in that it allows (1) injection of photogenerated holes from the charge generation layer and (2) efficient transport of these charges to the surface of the transport layer to discharge a surface charge thereon.
One class of charge transport molecules, N,N'-diphenyl-N, N'-bis(alkylphenyl)-[1,1'-biphenyl]- 4,4'-diamines, has been extensively studied in the forms of solutions or dispersions in polycarbonate polymers. The conductivity of this class of compounds in polycarbonate polymers has been found to increase under certain circumstances. When imaging members employing this class of compounds have a residual amount of a halogen-containing alkane solvent in the transport layer and they are subjected to ultraviolet radiation, a condition known as "cycle down" progressively develops as the device is cycled in the xerographic process. "Cycle down" refers to the progressive increase in conductivity of the transport layer such that in a relatively short period of time the charge acceptance of the device deteriorates.
Also, special precautions have to be taken with regard to the handling of the devices employing these compounds. They cannot be stored or left exposed to ambient room light for any length of time because fluorescent lamps employed in most buildings contain a UV component. This UV radiation causes devices left exposed to the room light to undergo gradual deterioration resulting in an increase in the conductivity of the transport layer. Chemical stabilization of the physical properties of polymers has been well pursued and additives are available that stabilize various polymers against ultraviolet induced discoloration or ultraviolet induced mechanical failures.
A photoreceptor is also subjected to a large number of chemical species produced by the charging devices typically used. A number of these species, especially the oxides of nitrogen, can react with hole transporting materials. This results in an electrically conductive surface. An electrostatic image residing on such a surface tends to spread, and in severe cases can totally desperse, producing a fuzzy to non-existant final image. By chemical stabilization, as used in the present invention, is meant the elimination or minimization of chemically induced conductive species on the surface of the photoreceptor.
To be useful as a stabilizer in a cyclic duplicating machine, the additive, in addition to preventing chemical degradation, has to meet another stringent requirement. The additive should not introduce traps, or conductive species of its own. Even a small number of traps results in the cumulative trapping phenomenon generally referred to as "cycle up."The trap could be an isolated electronic state of the additive or result from the additive changing the character of the dispersion of the host molecule in the binder matrix. The generation of conductive species on the photoreceptor surface would result in unacceptable image quality. This conductive species could be the oxidized state of the hole transporting moiety present in the photoreceptor transport layer.
It has been found that when some multi-layered drum imaging members are cleaned with flexible cleaning blades, the cleaning blade interacts with the drum imaging members in such a way as to cause a vibration in the drum. This vibration may be perceived as an irritating squeak. This squeak can be eliminated by plasticizing the charge transport layer. However, plasticization of the charge transport layer softens the layer and adversely affects its durability. In other words, the layer, subjected to the abrasive conditions generated by the interaction of the development materials and processes, tends to wear out too rapidly. Also, high loading of plasticizers may adversely affect the electrical mobility of the imaging member and interfere with other electrical and physical properties.
U.S. Pat. No. 4,297,425, to Pai et al., discloses an imaging member containing a hole transport layer comprising a polycarbonate resinous material having a residual amount of a halogen-containing organic solvent. Dispersed in the layer is a diamine as a charge transport compound and a diaryl or triaryl methane compound as a stabilizing compound to overcome effects of the residual halogen-containing solvent. The reference discloses a weight ratio of the stabilizing compound to the diamine compound of from 0.0005:1 to 0.1:1.
However, U.S. Pat. No. 4,297,425 does not disclose any relationship of the diaryl or triaryl methane compound to squeaking in a drum imaging member cleaned using a cleaning blade. The reference does not teach the use of diaryl or triaryl methane compounds in amounts greater than 0.1:1 relative to the diamine compound or selecting an electrophotographic system comprising a cleaning blade and a drum imaging member with a transport layer containing a diaryl or triaryl methane compound. It also does not teach an imaging member having a transport layer free from halogen containing solvents comprising a diaryl or triaryl methane compound.